1. Field of the Invention
The present invention relates generally to an electrochemical sensor used for the quantification of a specific component or analyte in a liquid sample. Particularly, the present invention relates to an electrochemical sensor for the detection of analytes present in biological fluids. More particularly, the present invention relates to a disposable electrochemical sensor for the in vitro detection of oxygen concentration. Still more particularly, the present invention relates to a method of correcting of the oxygen effect on oxidase-based analytical devices.
2. Description of the Prior Art
The first generation of oxidase-based electrochemical biosensors used oxygen as the electron acceptor. Oxygen, however, becomes a limiting factor in the enzymatic reaction at high substrate concentrations due to the limited solubility of oxygen in liquid samples. This limits the upper linear range of the biosensors. In order to overcome problems associated with insufficient oxygen concentration, a second generation of biosensors was developed that used electron mediators as substitutes for oxygen. The electron mediators are redox chemicals capable of mediating the electron transfer in the regeneration of the enzyme to the active form. The linear range of the resulting biosensors was significantly improved.
Various redox mediators have been investigated towards the development of commercial products. Among them, potassium ferricyanide, osmium redox couples, and ferrocene derivatives are the most popular. Such sensor reagent improvements highlight a key to the marketability of this type of product such as, for example, disposable glucose measuring strips. Disposable glucose measuring strips are based on the use of glucose oxidase and potassium ferricyanide, osmium redox couples, or ferrocene derivatives.
Although the mediator/oxidase-based biosensors eliminate the dependence on the oxygen concentration for the extended linear range of the sensor, oxygen related drawbacks still exist. Mediators are not as efficient at shuttling electrons with the enzyme as is the oxygen molecule. In fact, any oxygen in the sample solution can effectively compete for the enzyme site. Measurements made with the mediator/oxidase-based biosensors show significantly lower results with increasing oxygen in the fluid samples. The inaccurate testing results caused by varying oxygen concentration were investigated by several groups. This becomes especially important when using the glucose strips for point-of-care glucose testing in patients with high or unpredictable blood oxygen levels.
Additionally, biological specimens contain widely varying oxygen levels. The typical oxygen partial pressure, pO2, of a venous blood sample is about 32±7 mm Hg. In some cases, it can be as low as 20 mmHg and as high as 50 mm Hg. For a normal arterial sample and capillary sample, one can expect much higher oxygen levels of about 85±10 mm Hg. For patients who are in oxygen therapy, the level of arterial pO2 can reach 200 mm Hg or higher. Thus, the mediator/oxidase-based biosensors are prone to giving inaccurate test results resulting from the different oxygen concentrations when they are used for the measurement of blood samples other than capillary blood. This becomes more serious when the substrate concentration is at a low concentration level (e.g. glucose concentration less than 70 mg/dL).
To obviate the interference resulting from the varying oxygen concentration or so-called “oxygen effect” described above using the mediator/oxidase-based biosensors, an oxygen-insensitive enzyme such as glucose dehydrogenase was used to replace the oxygen-sensitive oxidase such as glucose oxidase. Glucose dehydrogenase, whose coenzyme is pyrroloquinoline quinone, does not interact with oxygen. Therefore, the resultant glucose sensor is unaffected by variable oxygen concentration in the sample. A few products have been developed and marketed using this enzyme. These included Accu-Chek™ Comfort Curve® by Roche Diagnostics, IN, USA, Freestyle® by TheraSense, Alameda, Calif., USA, and Ascensia® by Bayer Health Care, Mishawaka, Ind., USA.
The use of glucose dehydrogenase does overcome the problems associated with the oxygen effect. Glucose dehydrogenase, however, has other problems. One problem is that it is not as specific as glucose oxidase. Glucose dehydrogenase not only reacts with glucose but also reacts with other sugars like galactose and maltose. Both galactose and maltose have a similar structure to glucose. Maltose is composed of two glucose units and galactose differs in structure from glucose only in the position of the hydroxyl group on carbon number 4. Severe interference can be expected. As a matter of fact, the glucose dehydrogenase-based biosensors are more sensitive to maltose and have no discrimination between glucose and galactose. If a glucose monitor or test strips use a glucose dehydrogenase pyrroloquinolinequinone method, a falsely high glucose reading may be obtained by the patients. For this reason, the Centers for Medicare & Medicaid Services and ESRD Networks were alerted by the Food and Drug Administration (FDA) on Apr. 18, 2003, to a concern with peritoneal dialysis patients' glucoses while on Icodextrin Extraneal dialysis solution and the effects of falsely elevated glucoses because of the interaction of maltose. A false high blood glucose reading could cause the patients to be given more insulin than needed. Getting more insulin than needed can lower a patient's blood sugar unnecessarily and can cause a serious reaction including, but not limited to, loss of consciousness.
Therefore, what is needed is an oxygen sensor that can be used to measure dissolved oxygen accurately and precisely with a minimum quantity of sample volume. What is also needed is an oxygen sensor that is disposable. What is further needed is a disposable oxygen sensor and a method of correcting the oxygen effect on oxidase-based analytical devices. What is still further needed is a system for correcting the oxygen effect on oxidase-based analytical devices.